With my first SCP doing well I thought I'd move on to another, but this time I don't think the idea is polished enough to just go ahead and drop it into the wiki. I wanted to do a really short and simple one, but is the result too generic and bland for an SCP? Should there be more an element of danger? More anomalous properties? Have these tube things (I can't figure out if they have an official name or not) been used before?

I've had so many conflicting ideas for how it could be scarier or simply more mysterious, off the top of my head:

-Adults who have been inside will permanently see all children in the abnormal manner.
-Children who exit appear deformed permanently, but only to the perception of adults.
-Children who exit grow differently than a normal human from then on
-Adults or animals who enter along with kids appear to be killed by them, come out just fine
-Viewed inside, interior appears to be living material. Children show no sign of noticing.
-Nonhumans who enter are really, permanently deformed
-Structure bleeds when damaged
-One deformed child is always visible from outside, doesn't correlate to anyone inside
-Drop the child/adult differences entirely?

I considered a log recording children who were sent inside, revealing unsettling behavior, possibly interacting with things invisible to the researchers or acting out things the children couldn't have known about.

Also thought maybe when kids are sent in they encounter and play with other seemingly normal children only they can perceive, and those are the ones only visible from outside as "monsters."

Maybe when people over a certain age enter the structure they are deformed as seen in the picture?

Or maybe the thing "feeds" on children, so when they enter they get lost, and over time eventually degrade or something.

Maybe it feeds off of the happiness and general sense of well-being children typically have when they enter a playplace, and over time they become mutated monstrosities that are only visible to adults from the outside of the structure.

As is, it only appears to be doing anything at all, and is really just a sort of novelty.

All the ideas you're considering would work pretty well. You just need to pick the ones that work best together and developed them, like PureQuestion said. The SCP as it is right now is still creepy and interesting, but lacks something that would make it a complete work. This does have a lot of potential.